1. Field of the Invention
The present invention generally relates to the field of optical fibers and of the manufacturing thereof.
2. Description of the Related Art
Optical fibers, particularly glass fibers, are manufactured starting from a previously made glass body, usually called “preform”, by a process known in the art as “drawing”. A glass preform is placed at the top of a fiber drawing tower, where it is heated up in a furnace to a temperature sufficiently high to cause the softening of a bottom portion of the preform. The softened preform material is drawn by a tractor, to form an optical fiber glass core.
The glass core is surrounded by a layer, generally of glass, having a refractive index lower than that of the core, said layer being called cladding. In the following, the glass core surrounded by the cladding could be referred to as “optical waveguide”.
Over the cladding, at least one, more often two superposed UV radiation-cured coating layers are provided, which form the so-called coating system.
Usually, the coating system is applied onto the optical fiber core during the drawing process.
The coating layer which is in direct contact with the glass core is called “first coating” or “primary coating”; the overlaying coating layer, which is on the exposed surface of the coated fiber, is called “second coating” or “secondary coating”.
The coating system helps to absorb forces applied to the coated fiber, and subsequent losses associated therewith, provides protection against microbending, that can lead to attenuation of the signal transmission capability of the coated optical glass fiber, endows the fiber with the desired resistance to handling forces, such as those encountered when the coated fiber is cabled.
The first coating is usually a soft coating, having a relatively low elastic modulus. The second coating is typically a coating having a higher elastic modulus.
Color coding is typically used to distinguish and identify individual fibers in a complex cable.
For example, in telecommunications applications, multiple coated fibers can be arranged into larger structures, such as ribbons and cables, to maximize efficiency. However, after “ribboning” (i.e., arranging a number of fibers side by side and coating them with a common coating in the shape of ribbon) and cabling of the fiber, the individual fiber should be readily distinguishable from each other, so that they can be accurately identified during, for example, installation and maintenance.
Although several methods can be used to color code optical fibers, color coding can be done advantageously with either a colored layer (typically as thick as about 10 microns or less), which is placed over the coated fiber before cabling and/or “ribboning” of the same, or by applying a colored second coating onto the first coating.
The application of a colored layer can take place during the drawing process of the optical fiber.
Optical fiber ribbons are prepared by embedding at least two individual color coded fibers in a common matrix material which, like the first and second coatings, is also radiation-curable. Optical fiber ribbons may comprise, e.g., 4 to 12 colored fibers. The matrix material can encase the color coded optical glass fiber, or the matrix material can edge-bond the glass fibers together. Cure of the matrix material occurs during the “ribboning” stage, after the fibers have been color-coded by applying a colored layer.
In a ribbon design, the colored layer resides between the ribbon matrix material and the fiber second coating. This means that the interfacial characteristics (e.g., surface energy, adhesion) of the colored layer should be carefully selected to fit with those of both the matrix material and the second coating material in the ribbon structure. In particular, the ability of a cured matrix material to be suitably stripped off the colored layer (break-out) is an important technical consideration. Ribbon break-out is generally carried out by applying a mechanical force, although chemical softening of the matrix with use of solvents is also known.
The use of a colored second coating is disclosed, for example, in U.S. Pat. No. 6,797,740.
US20040170367 relates to optical fibers including a layer of primary coating material having a first modulus, a layer of color coating material having a second modulus, a layer of secondary material having a third modulus, and wherein the first, the second, and the third modulus values are different.
The layer of the primary coating material, the layer of the color coating material, and the layer of the secondary coating material are each applied prior to the other layers being cured. All of the three layers are cured together.
In an embodiment a coated optical fiber includes a layer positioned between the primary coating layer and the secondary coating layer. The primary coating layer surrounds the optical fiber (i.e. the optical waveguide), the colored coating layer surrounds the primary coating layer, and the secondary coating layer forms the outermost protective layer. Typically, the color coating material has a modulus between that of the primary and secondary coating materials.
In an alternative embodiment, the color coating layer surrounds the optical fiber and the primary coating layer is between the color coating layer and the outermost secondary coating layer. In this instance, the modulus of the color coating material is preferably less than or equal to the modulus of the primary coating layer. In an alternative embodiment, the primary coating layer is adjacent the optical fiber, the secondary coating layer is adjacent the primary coating layer, and the color coating layer is the outermost layer. The document provides no indication about the modulus values of the layers in this instance.
US20040179799 provides an optical fiber cable that includes a core comprising one or more optical fibers surrounded by a coating system (therein referred to as “protective sheath”), which has a radially-varying elastic modulus, and a method for making the same. The protective sheath includes first and second coating layer portions based on the same coating material. A modifier is added to the coating material of the first coating layer portion. Likewise, a modifier is added to the coating material of the second coating layer portion. The addition of a modifier to the first coating layer portion and the addition of a modifier to the second coating layer portion cause the coating system to have an elastic modulus that varies in a radial direction along radii extending outwardly from a center of the core of the cable. Different types of modifiers that can be used for this purpose include, but are not limited to, fillers, e.g. nanoclays; cross-linking agents, e.g. acrylates; polymerization chain transfer agents; photoinitiators, e.g. alpha-hydroxy ketones. The coating is provided with different modulus by adding different amounts of modifiers or different modifiers to the coating material.
In an example, the radially-varying elastic modulus varies gradually. The radial variation could be, for example, a step-wise function such that the radial variation would change abruptly at some location within the coating.
The optical fiber may comprise an additional coating that surrounds the outer portion of the coating (three layers). The optical fiber may further comprise a color layer, such as ink, for example, which surrounds coating layer.